1. Field of the Invention
The present invention relates to a method for toe angle adjustment for a vehicle and a toe angle adjusting apparatus therefor.
2. Description of Related Art
A toe angle checking step is provided downstream in a vehicle assembly line for vehicles, in which a final adjustment is performed for a toe angle of the wheel (an angle between the plane of the wheel and the vertical plane). At this end, the wheels of the vehicle are provided each with a toe angle adjusting mechanism. Conventionally, for example, for the front wheels, such a toe angle adjusting mechanism is usually mounted on a tie rod that is one mechanism element for a steering mechanism. Various types of toe angle adjusting mechanisms are known. Japanese Utility Model Application (laid-open) No. 103,005/1985 discloses a toe angle adjdusting mechanism of a type using a toe angle adjusting rod with a threaded portion. More specifically, the toe angle adjusting mechanism of this type is such that, for example, when the toe angle adjusting rod is mounted on the tie rod, the tie rod is arranged so as to extend or shorten its lengthwise dimension by a length corresponding to a screwed pitch of the threaded portion thereof when the toe angle adjusting rod makes one full revolution, thus shortening the toe angle in a toe-in or toe-out direction and changing a direction in which the vehicle advances.
At the toe angle checking step in the vehicle assembly line, there are performed a horizontal adjustment of a steering wheel, or adjustment of the steering wheel so as take a horizontal position, or a neutral position, and the mounting of the steering wheel, in addition to adjustment for toe angles of right-hand and left-hand front wheels as well as right-hand and left-hand rear wheels.
The toe angle adjustment has been performed on the basis of a result of measuring tire angles of the wheels. Measurement of tire angles may be made by various known techniques, such as the technique involving operation of a tire angle from a side force of each wheel rotating on a drum using a so-called dynamic toe tester or the technique, as disclosed in Japanese Patent Publication (laid-open) No. 100,307/1982, involving a direct determination of a tire angle from an angle of inclination of a measuring plate disposed so as to abut with an outer surface of the vehicle. In such conventional toe angle adjustment, a toe angle of each wheel has been adjusted by an operator on the basis of a toe angle adjusting amount displayed on a display screen. The toe angle adjusting amount, however, has been determined on the basis of a device for measuring a toe angle of the wheel, or a tire angle measuring device. More particularly, a center line extending in a longitudinal direction of the vehicle cannot be measured so that a reference line formed by the tire angle measuring device is regarded as a center line of the vehicle and a tire angle of the wheels obtained upon measurement by means of the tire angle measuring devices is regarded as a toe angle thereof. This technique may offer the advantage that the front and rear wheels can be adjusted on the basis of the same reference line.
However, it has been necessary to check every wheel as to whether the toe angle adjustment should be performed and, as a matter of course, if it has been found the toe angle adjustment required for all the wheels, the toe angle adjustment has been performed for all the wheels. It is apparently understood that the toe angle adjustment for all the wheels is very laborious and, even if the adjustment would be rendered automatic, a toe angle adjusting device should be disposed for every wheel.
Japanese Patent Publication (Laid-open) No. 48,875/1986 discloses an example of automation of the toe angle adjustment which involves adjusting a tire angle of the wheel so as to reach a predetermined target toe angle (as a reference value) on the basis of a found tire angle measured by a tire angle measuring device, the found tire angle being regarded in this example as a toe angle. In other words, this example performs the toe angle adjustment on the basis of the tire angle measuring device. In this case, the vehicle is provided with a toe angle adjusting mechanism consisting of a cam bolt with an eccentric cam, the cam bolt being designed so as to be rotatable about its axis leading to the toe angle adjustment. Accordingly, a toe angle adjusting amount that can be adjusted by this prior embodiment should become very small. A toe angle adjusting device basically comprises a mechanism for rotating a cam head.
Another technique is disclosed in Japanese Patent Publication (Laid-open) No. 100,307/1982, which regards a tire angle on the basis of a composite angle of the left-hand and right-hand wheels as a toe angle, while the technique disclosed in Japanese Patent Publication (Laid-opne) No. 48,875/1986 regards the found tire angle on the basis of the tire angle measuring device as a toe angle.
The composite angle disclosed in Japanese Patent Publication (Laid-open) No. 100,307/1982 is defined as follows: ##EQU1## (where) .theta.R=a found tire angle of the right-hand rear wheel measured by the tire angle measuring device; and
.theta.L=a found tire angle of the left-hand rear wheel measured by the tire angle measuring device, provided, however, that one of .theta.R and .theta.L is represented by positive sign (+) and that the other is represented by negative sign (-). PA1 .theta.L=a found tire angle of the left-hand front wheel measured by the tire angle measuring device, provided, however, that one of .theta.R and .theta.L is represented by positive sign (+) and that the other is represented by negative sign (-).
The composite angle of the rear wheels means a direction in which the vehicle advances straight, a direction being decided on the basis of a relative relationship of the right-hand rear wheel with the left-hand rear wheel. Thus it is to be understood that the found tire angle is corrected on the basis of the composite angle of the rear wheels and that the corrected tire angle is regarded as a toe angle. The toe angle for the right-hand rear wheel may be computed as follows: ##EQU2##
A composite angle of front wheels may be defined as follows: ##EQU3## (where) .theta.R=a tire angle of the right-hand front wheel measured by the tire angle measuring device; and
For a toe angle for the front wheel, like the toe angle for the rear wheel, the tire angle measured is corrected on the basis of the composite angle of the front wheels in substantially the same manner as that measured for the rear wheels, and the corrected tire angle is regarded as a toe angle for the front wheel.
The above technology is based on adjustment for the toe angle of the front wheels on the basis of the composite angle of the front wheels, independently from adjustment for the toe angle of the rear wheels on the basis of the rear wheels. This technique permits a uniformity in the toe angle adjusting amounts for both the left-hand and right-hand front wheels as well as for both the left-hand and right-hand rear wheels. However, this technique likewise requires the toe angle adjustment for all the wheels. If there is a deviation between the composite angle of the front wheels and the composite angle of the rear wheels, the front wheels and the rear wheels are adjusted on a different refernce line.
If the toe angles of the front and rear wheels are deviated from referece values, the corresponding wheel or wheels is or are subjected to the toe angle adjustment for correction.
Japanese Patent Publication (Laid-open) No. 100,307/1982 discloses adjustment of a steering wheel to a horizontal position, that, adjustment so as to allow the steering wheel to take a horizontal position, or a neutral position, when the vehicle advances straight. This adjustment will sometimes be called horizontal adjustment in the following description. In the disclosure in this prior patent application, the horizontal adjustment is designed so as to be performed by adjusting a state in which the steering wheel should be mounted. More specifically, the steering wheel is mounted so as to take a state in which the steering wheel is steered by an amount, .alpha., of deviation of the composite angle of the front wheels from the composite angle of the rear wheels by an operator who is visually confirming such an amount .alpha. with the assumption that the deviation between the composite angles adversely affects a horizontal position at which the steering wheel is held.
The amount of deviation, .alpha., may be defined as follows: ##EQU4##
It should be noted that the above technique is adopted with a premise that the front wheels are in a neutral state.